Techniques and apparatus for pulling single crystal bodies from a melt of the material have been known for some time and are constantly being improved. Bodies such as sapphire tubing drawn from a melt of alpha-alumina or semiconductor articles drawn from silicon or germanium melts are examples of industrial use of such apparatus and Technique. Typical furnaces for growing crystals from the melt are shown in U.S. Pat. Nos. 3,471,266 and 3,679,370, for example.
Inherent in the process of crystallizing from the melt is the need to replenish the melt batch as it is depleted. However, several problems which are variously found in prior art apparatus must be overcome in order to effectively supply the system with batch material. One problem is that the melt, which is held in a crucible in the crystal growing furnace, must be maintained at a very constant temperature for good crystal growth. New batch material must therefore be delivered at a constant rate to avoid wide flucuations in temperature in the crucible with each entry of make-up material. The temperature of the batch material may also be raised by pre-heating to avoid introducing large temperature gradients due to large temperature differences between batch and melt.
A second problem is one of contamination of the make-up material before its addition to the crucible. Poor ware is inevitable if the raw batch picks up impurities in the air and carries them to the crucible. For example, when pulling sapphire tubing, the .alpha.-alumina batch may pick up water from the air, which if present in the melt, can cause excessive voids in the tubing.
The answer to the second problem may bring on the third problem. To avoid the impurities, the batch material may be heated in an air-evacuated preheating furnace independent of, but in communication with, the crystal growing furnace. The batch material is placed in the preheat furnace, the temperature is raised, and the furnace is outgassed and optionally backfilled with an inert atmosphere.
The problem brought on by the addition of a preheat furnace in the system is that the material must now be transported from the preheat furnace at a constant and uniform rate to the crucible in the crystal growing furnace, the whole system being maintained gas-tight. The feeder mechanism must now also apparently operate in a high-temperature environment, one not conducive to the long life and efficient operation of feeder moving parts and especially of bearings which may generally be necessary between moving parts.